Google will send password resetting instructions to the secondary email address you provided during GMail signup.

If you don't have a secondary email address, try the same link again after five days. At that point, you'll be able to reset your password by answering the security question you provided when you created your account.

Though there is little you can do in situations when cannot answer the GMail security question and do not have access to the alternate email address, here's the link to contact GMail support. (choose Login/Account access from the topic list)